This invention relates to methods and apparatus for gamma-ray spectroscopy and like measurements of high-energy photons in boreholes traversing earth formations.
Borehole logging using detection of high-energy photons such as gamma-rays and X-rays, with or without spectral analysis, is an important technique in the discovery and development of subsurface reserves of hydrocarbons. Typically, after a borehole has been drilled an elongate logging tool or sonde is lowered to the bottom of the borehole on an armored cable containing electrical conductors for transmitting electrical power and signals between surface equipment and the sonde. The sonde is then raised up the borehole by winching the cable, and detectors in the sonde are used to sense high-energy photon radiation either naturally emitted by radioactive elements in the formations (such as uranium, thorium and potassium) or resulting from irradiation with penetrating radiation emitted by a source contained in the sonde.
If a source is provided the radiation may take the form for example of neutrons, gamma rays or X-rays, and may originate from decay of atoms (involving either a nuclear or an atomic transition) in a radioactive material or be generated artificially by an electrically-powered accelerator. Examples of possible sources are cesium-137 which coninuously emits gamma radiation in the course of decaying to form barium, and the neutron accelerator described in U.S. Pat. No. 3,461,291 to Goodman and U.S. Pat. No. 3,546,512 to Frentrop which emits neutrons either continuously or in pulses depending on the manner of operation. The radiation thus emitted by the sonde interacts with the atoms of the materials in the vicinity of the sonde (e.g. the borehole contents and/or the constituents of the formations), and is modified in a manner dependent on the characteristics of those materials. The detectors in the sonde may sense either the scattered and attenuated photon (gamma or X-ray) radiation in the case of a photon source, or gamma radiation induced by interaction with neutrons produced by a neutron source. Yet another possibility is the detection of activation gamma radiation resulting from decay of radioactive isotopes artificially induced in the formation by irradiation with neutrons; such activation radiation typically persists for periods of the order of minutes after irradiation.
Borehole detectors of high-energy photons such as gamma rays and X-rays commonly employ a specially grown scintillation crystal which produces a burst of visible or near-visible photons (i.e. relatively lower energy photons) when a high-energy photon interacts with its constituent atoms. This burst or light flash is sensed by a photomultiplier adjacent the scintillation crystal to produce an electrical signal indicative of the flash intensity, which is dependent upon the energy of the incident high-energy photon.
Parameters of the detected gamma rays or X-rays such as their energy and/or rate of occurrence are measured and the measurements transmitted via the armored cable to the surface equipment to be recorded. Analysis of, for example, the time and/or energy spectrum distributions of the detected high-energy photon radiation enables information about subsurface conditions to be deduced to aid the detection and development of subsurface hydrocarbon deposits.
The environment in a borehole is hostile to electrical and mechanical equipment, involving wet or corrosive surroundings and possibly high pressures and temperatures. consequently a borehole logging sonde must be of rugged design. Furthermore the space in a borehole is very confined--in the case of logging sondes designed to pass through production tubing the outer diameter is limited to 1 11/16 inches (.sup..about. 42.9 millimeters). These requirements impose severe constraints on the materials and construction of a logging sonde. In addition the maintenance and operation of a borehole drilling rig is extremely expensive, so time spent on activities other than actual drilling (e.g. borehole logging) must be minimized.
One particular problem in the design of borehole high-energy photon detectors is the choice of scintillation crystal. This should ideally have several specific properties, such as: a high density to maximize the possibility of interactions between high-energy photons and the scintillator material, and thus the efficiency of detection of such photons traversing the crystal; high effective atomic number to provide a high photo-peak efficiency, that is maximize the possibility of the desired photo-electric interaction and minimize undesirable Compton scattering which reduces gamma ray and X-ray energy and degrades the accuracy of the determination of incoming photon energy; fast decay of the scintillation process following a gamma ray of X-ray interaction and low afterglow; high scintillation light output linearly related to incoming photon energy to provide good energy resolution and accurate energy measurement; high transparency to minimize attenuation of the light flashes by the scintillator material; insensitivity of the scintillation process to temperature changes; inert behavior in the presence of materials typical of the borehole environment, and good mechanical strength; availability in sizes of the order of several centimeters in length and diameter. Many candidate materials for use as borehole scintillators fail to provide one or more of these properties.
The material most commonly used at present for logging applications is sodium iodide doped with thallium. This material has several disadvantages that have imposed undesirable constraints on logging operations: it has a low density, so its detection efficiency is low; it has a slow decay of the scintillation process, limiting the rate at which it can respond to high-energy photon interactions; the afterglow is large and persists for a long time after the interaction of a photon with the crystal; and it is hygroscopic, so it must be enclosed within a hermetically sealed housing. Although sodium iodide has some advantageous characteristics, including a high light output, good energy resolution and availability in relatively large sizes, and its sensitivity to temperature changes can be compensated relatively easily, the use of sodium iodide has restricted logging operations undesirably in terms of low logging speeds, poor measurement statistical reliability and failure of hermetic sealing for example.
Another material that has been investigated recently is bismuth germanate (BFO). This has a much higher density than sodium iodide, a high effective atomic number owing to the presence of bismuth, and it is not hygroscopic. Unfortunately, its light output is considerably lower than that of sodium iodide (around 88% lower) and furthermore the light output drops precipitously with increasing temperature. Hence some kind of thermal stabilization is necessary such as active cooling or enclosing the scintillator in a dewar. Unfortunately, the material in the walls of such a dewar attenuates incoming radiation undesirably, and consumes space thereby decreasing the maximum size of the crystal, which in turn leads to restrictions in logging speed, statistical reliability, etc. This renders the incorporation of BGO in practical slim logging tools very difficult or impossible. In addition the spectrum of its light output is not well matched to common existing photomultiplier tubes, so development of new photomultiplier technology would be required to take full advantage of its desirable properties and offset the effect of its inherently low light output. These problems are made worse by the relatively high refractive index of BGO, which results in a tendency for light to be trapped in the crystal by total internal reflection. Furthermore the scintillation decay time of BGO is even longer than that of sodium iodide, leading to greater problems with pulse pile-up.
It is an object of this invention to provide methods and apparatus for gamma ray spectroscopy and like measurements which alleviates problems associated with the use of scintillation materials in the difficult and hostile environment encountered in borehole logging.